Set global.flannel.uninstallOnExit=true if you want Cilium to uninstall
itself when the Cilium pod is stopped.

If the Flannel bridge has a different name than cni0, you must specify
the name by setting global.flannel.masterDevice=....

Optional step:
If your cluster has already pods being managed by Flannel, there is also
an option available that allows Cilium to start managing those pods without
requiring to restart them. To enable this functionality you need to set the
value global.flannel.manageExistingContainers=true

Once you have changed the ConfigMap accordingly, you can deploy Cilium.

kubectlcreate-fcilium.yaml

Cilium might not come up immediately on all nodes, since Flannel only sets up
the bridge network interface that connects containers with the outside world
when the first container is created on that node. In this case, Cilium will wait
until that bridge is created before marking itself as Ready.